1. Field of the Invention
The invention relates, in general, to semiconductor microelectronic sensors, and more particularly, to single crystal silicon sensors that include structures with diverse contours and high aspect ratio geometries.
2. Description of the Related Art
The electrical and mechanical properties of silicon microsensors have been well chronicled. For example, refer to Kurt E. Petersen, "Silicon as a Mechanical Material", Proceedings of the IEEE, vol. 70, No. 5, May 1982. Moreover, there is a large and growing body of knowledge concerning techniques for constructing silicon microstructures, commonly referred to as "micromachining". See, for example, Bryzek, Petersen and McCulley, "Micromachines on the March", IEEE Spectrum, May 1994, pp.20-31.
Silicon micromachining has blossomed into a vital industry with numerous practical applications. For instance, micromachined silicon pressure and acceleration sensors have found their way into medical instrumentation and automobiles. The high strength, elasticity and resilience of silicon makes it an ideal base material for resonant structures that may, for example, be useful for electronic frequency control. Even consumer items such as watches, scuba diving equipment, hand-held tire pressure gages and inflatable tennis shoes may soon incorporate silicon micromachined sensors.
The demand for silicon sensors in ever expanding fields of use continues to fuel a need for new and different silicon microsensor geometries optimized for particular environments. Unfortunately, a drawback of traditional bulk silicon micromachining techniques has been that the contours and geometries of the resulting silicon microstructures have been significantly limited by these fabrication methods. For example, anisotropic etching of single crystal silicon (SCS) can achieve an anisotrophy rate of 100:1 in the &lt;100&gt; crystallographic direction relative to the &lt;111&gt; direction. The result of such anisotropic etching of SCS, however, typically will be a silicon microstructure with sidewalls that are inclined because of the intersection of the (100) and (111) crystallographic planes. As a result, the contours of silicon microstructures have been limited by the orientation of the internal crystallographic planes. Thus, there has been a need for silicon microsensors having structures with more diverse geometric contours.
The increasing use of microsensors to measure pressure and acceleration has spurred the development of tiny silicon plate structures used as capacitors and to produce electrostatic forces, for example. For instance, there exist microsensors that measure capacitance using an array of interdigitated polysilicon plates. Similarly, there exist microsensors that produce electrostatic forces using an array of interdigited plates. Ordinarily, the surface areas of such plates are relatively small since they typically are formed in a deposited polysilicon layer. Increasing the surface area of such capacitive plates increases their capacitance. Increasing the surface area of such electrostatic drive plates increases their drive capability. Hence, there has been a need for capacitive plates and electrostatic drive plates with increased surface areas.
There also is a need for improved silicon microstructures on which electronic circuitry can be formed. For example, metal oxide semiconductor (MOS) circuits generally are most effective when formed in (100) silicon wafers. Unfortunately, traditional silicon micromachining techniques usually favor the formation of microsensors in (110) wafers. Hence, MOS circuits have not been prevalent in silicon microsensors. Moreover, in some applications there can be a need to thermally isolate a circuit formed as part of a microsensor in order to ensure optimal circuit performance.
A problem with tuneable resonant microstructures formed from materials such as polysilicon or metal is that they can suffer frequency drift over time due to internal crystal stresses that develop from usage. Thus, there is a particular need for a microstructure that employs a high-Q resonator that does not suffer from crystal stresses. It has long been known that SCS is an excellent base material for a resonant structure. It is strong, flexible and highly elastic, and its single crystal structure makes it more resistant to performance degradation. However, tuning the resonant frequency of an SCS resonant structure can be a challenge. Consequently, there is a need for an improved approach to the tuning of a high-Q SCS resonator.
Thus, there has been a need for silicon microsensors that incorporate structures with more diverse geometries including structures with contours that are not limited by the crystallographic planes of silicon and plates with increased surface areas. There also has been a need for silicon microsensors with structures that are better suited to the formation of electronic circuitry. In addition, there has been a need for silicon microstructures with improved resonant structures. The present invention meets these needs.